When refrigeration heat pump compressor apparatus is used to supply heat to a space such as a home, many heat pump manufacturers require that the heat pump compressor be turned off at certain low outdoor temperatures. To heat the space during these times only, a more expensive auxiliary heat apparatus such as electric strip heaters is used to maintain the temperature control in the space. The control of the heat pump apparatus and the auxiliary heat is generally accomplished by a thermostat with at least two stages. The first stage is used to control the heat pump apparatus and the second stage is used to control the auxiliary heat apparatus. A differential temperature exists between the two stages of control. In a forced air heating system, the fan for delivering conditioned air to the space is often turned on and off with the first stage during the control of the heat pump apparatus. An outdoor thermostat is used to prevent the operation of the heat pump compressor below some outdoor temperature, and at that predetermined outdoor temperature, the first stage control of the thermostat is transferred to the control of the auxiliary heating apparatus and the heat pump is rendered inactive.
The control temperature of the first stage ranges from 0 to 100% load along a characteristic curve known as the first stage temperature droop curve. The second stage of the thermostat has a temperature droop curve throughout the 0 to 100% load of the second stage; however, the curves are offset by the temperature differential between the stages. When the control of the auxiliary heat is switched from the second stage to the first stage, a substantial change in the control temperature takes place which is often unacceptable to the home owner.
The present invention is concerned with the control of the heat anticipation to the thermostat. While each stage has an anticipation means, when the control of the auxiliary heating apparatus is switched to the first stage, the anticipation means for each of the stages is energized by the first stage. With the additional anticipation, the temperature droop curve for the first stage has a greater droop and the control point of the first stage is not greatly different than the control point of the second stage to overcome the objection of a drastic change in the control point when the switchover occurs.